The following meanings for the abbreviations used in this specification apply:
CB Coordinated Beamforming
CoMP Coordinated Multipoint
CRS Common Reference Signal
CS Coordinated Scheduling
CSI Channel State Information
DL Downlink
DPS Dynamic Point Selection
eNB Enhanced Node B. Name for Node B in LTE
E-PDCCH Enhanced Physical Downlink Control Channel
FDD Frequency Division Duplex
ID Identity
JT Joint Transmission
LTE Long Term Evolution
LTE-A Long Term Evolution Advanced
MIMO Multiple-Input Multiple-Output
MU Multi User
OFDM Orthogonal Frequency Division Multiplexing
PCFICH Physical Control Format Indicator Channel
PDCCH Physical Downlink Control Channel
PDSCH Physical Downlink Shared Channel
PRB Physical Resource Block
RAN Radio Access Network
RRC Radio Resource Control
RRH Remote Radio Head
SI Study Item
SU Single User
TDD Time Division Duplex
UE User Equipment
UL Uplink
UL-SCH Uplink Synchronization Channel
WI Work Item
In the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) and LTE-Advanced (LTE-A), single cell single-user (SU-) and multi-user (MU-) multiple-input multiple-output (MIMO) network performance is interference-limited, especially at the cell edge. Therefore, introduction of the technology of coordinated multipoint (CoMP) transmission/reception has been considered, where in downlink, multiple points co-operate in scheduling and transmission in order to strengthen desired signal and mitigate inter-cell interference. According to the 3GPP technical report on CoMP, TR36.819, a point is a set of geographically co-located transmit antennas and the sectors of the same site correspond to different points. It should be noted that a cell is formed by one or multiple points.
After RAN1#66 meeting the CoMP TR was approved. The agreed CoMP WID proposes the following focus for the CoMP work during Rel-11:
“The work for specifying CoMP support in Rel-11 should focus on                Joint transmission        Dynamic point selection, including dynamic point blanking        Coordinated scheduling/beamforming, including dynamic point blanking”        
In joint transmission (JT) CoMP two or more points transmit simultaneously to a CoMP user. Dynamic point selection (DPS) refers to a scheme where the transmission point is switched according to changes in signal strength. In coordinated beamforming/scheduling (CB/CS) the scheduling decisions of neighbor points are coordinated in order to reduce interference. In principle all schemes may include blanking/muting which means that one or more transmission points are blanked/muted to decrease the interference.
The agreed CoMP WI targets specification of intra- and inter-cell DL CoMP schemes operating in homogeneous and heterogeneous configurations. Four main scenarios have been studied so far: intra-site (scenario 1), inter-site with high power RRH (scenario 2), low power RRH within the coverage of the macro cell, without and with the same cell ID (scenarios 3 and 4, respectively). CoMP WI addresses both FDD and TDD, hence unified solutions should be targeted.
In the following, a problem which may occur in these configurations is described in more detail.
For CoMP, UE is configured with a CoMP measurement set which is the set of CSI-RS resources (transmission points, each mapped to one CSI-RS resource) that the UE is supposed to measure for CSI feedback. Additionally, a CoMP reporting set has been defined—this is the set for which the UE is reporting CSI feedback. Typically it is assumed that CoMP measurement set would be equal to the CoMP reporting set, however it may also be that it will be defined that the UE will have to downselect the points for which CSI is to be reported. In this case CoMP reporting set would be a subset of the CoMP measurement set. Finally, a CoMP cooperating set has been defined as the set of points actually transmitting to the UE. Also this set would be typically a subset of CoMP measurement/reporting set. The UE is not aware of the cooperating set as cooperating set is essentially a network implementation issue and full freedom is left for the eNB to decide on which points should be participating the CoMP transmission. The main problem addressed with this invention arises from the fact that the UE is not aware of the transmission points actually transmitting to the UE in a particular time/frequency resource (e.g. PRB pair). Furthermore, in addition to the transmitting points, the actual CoMP scheme might be transparent to the UE, even though the feedback would most likely be optimized for a certain specific scheme (or multiple schemes). CoMP schemes are described at high level in TR 36.814.
In addition to PDSCH transmissions with CoMP, each point will need to be transmitting the cell-specific transmissions relating to its cell ID. These transmissions include:                Physical downlink control channel transmissions (PDCCH)        Cell-specific reference signal transmissions        
PDCCH region size can typically take 1-3 OFDM symbols of the subframe (2-4 symbols in case of 1.4 MHz bandwidth; 0 symbols may become also possible in Release 11 for PDCCH-less (extension) carriers). The PDCCH region size may further vary on a per subframe basis—the length of the PDCCH region is indicated dynamically to the UE via the PCFICH channel. In CoMP, the problem becomes that the PDCCH region may be different in each cell participating in the CoMP transmission, and the UE will not know the PDCCH region size in each cell. It is noted that it is infeasible to assume that the UE would detect the PCFICH from each cell. Hence, UE will not be aware of the PDCCH region size used in each cell participating in the CoMP transmission. Optimally, such knowledge would be needed as follows:                For joint transmission, PDSCH can only be transmitted within the OFDM symbols that are not used in any of the cells for PDCCH.        For dynamic point selection, PDSCH can in principle be transmitted within the OFDM symbols that are not used in the same PRB(s) for PDCCH transmission from the selected point. However, this approach might lead to rather complicated PDSCH resource mapping, hence in practice it would be simplest to assume that PDSCH is mapped only to OFDM symbols in which none of the selected points is transmitting PDCCH.        For coordinated scheduling/beamforming, PDSCH is transmitted only from one point at a time. Hence, PDSCH would be optimally mapped to OFDM symbols not containing PDCCH from serving point.        For single point fallback mode, PDSCH would be mapped according to the serving cell PDCCH region (and CRS shifts, see below).        
Since the UE does not know the CoMP scheme, the transmitting points or the PDCCH region size in different cells, in practice the UE would have to be configured with the PDSCH starting position (symbol). Here the problem arises that such an approach will waste a lot of resources as the configuration will have to be done for the worst case; especially when transmitting only from a single point it would be much better in terms of overhead to follow the PDCCH region size of that point in PDSCH resource mapping.
A similar problem arises with common reference signals (CRS). In Release 8, the CRS pattern is shifted in frequency according to the cell ID. There are six different shifts possible, and if two or more Tx antennas are used, essentially three shifts result in non-overlapping CRS (since CRS for 2Tx are multiplexed within the same symbol). Again, since the UE is not aware of either the CoMP scheme or the transmitting points (or participating cells, hence CRS), the overhead will basically have to be accounted for according to the worst case. For example, one might configure the UE to avoid all CRS shifts corresponding to the cell IDs within the configured CoMP measurement set. And again the PDSCH overhead will be significantly increased.
An example configuration is shown in FIG. 5 with three cells (2Tx CRS each) configured in the CoMP measurement set, the cells having different PDCCH region sizes and different CRS shifts. Note that each of the cells may correspond to multiple points and not all of the points under each cell need to be part of the CoMP measurement set. Hence the set of cells here is the set of cells from which at least one point is part of the CoMP measurement set.
According to the worst case configuration, the PDSCH mapping would then be as shown in FIG. 6, i.e. PDCCH region is two OFDM symbols and all CRS shifts are avoided. On the other hand, FIG. 7, FIG. 8 and FIG. 9 illustrate the PDSCH mapping in case of ideal knowledge about CRS shifts and PDCCH region sizes. Furthermore in case of fallback to single point transmission the PDSCH mapping could ideally follow the PDSCH mapping according to the serving cell PDCCH region and CRS shifts.
In detail, FIG. 6 shows the worst case overhead, wherein PDSCH is mapped around CRS shifts and PDCCH of all according to a semi-static configuration only. FIG. 7A shows a case in which cells #1 and #2 are used for joint transmission CoMP with optimized PDSCH mapping configuration, and FIG. 7B shows the case in which cells #2 and #3 are used for joint transmission CoMP with optimized PDSCH mapping configuration.
FIG. 8 illustrates an optimum overhead in case of dynamic point selection. In the first PRB pair, one of cell #3 transmission points is transmitting, and in the second PRB pair, one of cell #2 transmission points is transmitting. FIG. 9 illustrates an optimum overhead with CS/CB where the serving point belongs to cell #1. Note that also fallback to single point transmission would ideally follow a similar PDSCH resource mapping.
So the problem is essentially, how to signal efficiently to the UE as much information as possible about the PDSCH resource mapping in order to optimize resource utilization in CoMP transmissions. It is noted that the problem could be circumvented by forcing operators to deploy CoMP such that PDCCH region as well as CRS shifts are the same in all cells. However, it is not clear how such an artificial restriction could be enforced in practice, or whether it is even feasible to specify something for that purpose such that the UE could in fact assume same shifts and PDCCH region sizes in implementation. From that perspective some signaling will be needed. It is also considered here that dynamic signaling of full information is not feasible due to control information size/overhead restrictions.
The issue has been discussed in 3GPP RAN1. during the LTE-Advanced study item (2008-2009). The basic proposed solutions are:                Semi-static (RRC) signaling of cell IDs for which the UE needs to avoid CRS shifts, and semi-static signaling of PDSCH starting position (symbol). This would correspond to the worst case as discussed in the previous section.        Simple dynamic signaling of which CRS shifts need to be avoided. Dynamic signaling to indicate PDSCH starting position (e.g. 4 options assuming that PDSCH can start from symbols 0 to 4). Such signaling would require quite many bits on PDCCH and therefore is not desirable.        
Thus, according to the prior art as described above, necessary configuration etc. for CoMP transmission might lead to an increased overhead regarding the required control signaling.